The present invention relates to a method and apparatus for introducing hot process or flue gases through an inlet duct into a gas cooler. The method and apparatus according to the invention are especially suitable for feeding hot gases as fluidizing gas into a gas cooler provided with a fluidized bed.
Hot process gases usually contain fouling components, such as fine dust and molten or evaporated components, which turn sticky when they cool and condense, thereby adhering to each other and to surfaces in contact with the gases. In this way, these fouling components may very fast grow harmful deposits on the wall surfaces in contact with the process gases. Usually, the deposits seem to accumulate most easily in the border area between the hot and the cooled surfaces. For example, gas inlets of waste heat boilers are places where such deposits usually accumulate. Consequently, the inlet becomes easily clogged unless swept at times. Sweeping as such may be difficult in those hot conditions.
Furthermore, it is normally difficult to disengage the deposits accumulated in the hot inlet opening because the deposits accumulating on hot surfaces are hard and compact. In most cases, the inlet ducts are of refractory-lined construction or of ceramic material, having a slightly uneven and possibly even porous surface, which contributes to the adhesion of deposits to the surfaces. Sweeping of a refractory-lined surface may in turn damage the refractory lining.
The formation of deposits has been attempted to prevent, e.g., by blowing gas which is, for example, recirculated, cooled and purified process gas, into the inlet. This prevents, to some extent, sticky compounds from adhering to the walls in the vicinity of the inlet. However, the volume of the recirculated gas has to be considerably large in order to keep the inlet clear. This enlarges the overall gas volume entering the gas cooler, which grows the dimensions of the gas cooler and subsequent gas cooling means, in other words, increases the costs. Furthermore, the efficiency of heat recovery from the gases is lowered by mixing of cooled gas with hot process gases prior to heat recovery units.
An object of the present invention is to provide an improved method and apparatus for introducing hot process gases into a gas cooler in comparison with those described hereinabove.
An object is especially to provide a method and apparatus by which the deposits accumulated in the hot gas inlet duct are readily removable.
A still further ojject is to provide a method and apparatus by which the properties of the deposits accumulated in the inlet duct allow such deposits to be readily disengaged from the duct walls.
A characteristic feature of the method according to the invention for introducing hot process or flue gases into a cooling chamber is that the inlet duct wall is indirectly cooled with a cooling medium by bringing the wall surface opposite to the gas side surface into contact with the cooling medium, whereby the deposits formed on the wall surface on the inlet duct gas side embrittle and become readily removable.
For disengaging the deposits from the inlet duct walls, these walls are subjected to a sudden mechanical force, which causes a temporary deformation or vibration of the wall, thereby loosening the deposits accumulated on the wall surface.
A characteristic feature of the apparatus according to the invention for introducing hot process or flue gases into a gas cooler is that the inlet duct of the gas cooler is formed of a cooled, elastic structure, in which the inlet duct walls are formed of cooled surfaces made of metal.
The inlet duct is preferably provided with an apparatus by which the inlet duct walls may be subjected to a sudden mechanical force, which causes a temporary deformation and/or vibration of the walls.
The invention is especially suitable for plants where hot process gases are cooled in a cooling chamber provided with a fluidized bed and where the hot process gas simultaneously serves as a fluidizing gas. In this case, the inlet duct is arranged in the bottom of the cooling chamber and hot gases are introduced into the fluidized bed via an inlet arranged in the bottom of the cooling chamber. Cooling is most preferably effected in a gas cooler provided with a circulating fluidized bed, where hot gases are introcuded into a mixing chamber and mixed with recirculated, cooled particles, whereby the gases cool very fast.
If the inlet duct is too short, particles may flow from the fluidized bed of the cooling chamber downwardly to the inlet duct with harmful results. Some turbulence is formed in the inlet, between the inlet duct and the cooling chamber, when the particles flowing downwardly along the cooling chamber walls meet the hot gases. The particles may thus flow downwardly into the inlet duct. From the inlet duct the particles are, however, carried away by the hot gases back to the cooling chamber provided that the inlet duct is of a certain minimum length. The ratio of the inlet duct length to the inlet duct diameter L/D has to be at least 0.5, preferably 1 to 2. For example, plants with the gas flow of 1000-200,000 Nm.sup.3 /h which are equipped with an approximately 5 to 30 m high gas cooling reactor provided with a fluidized bed and having a mixing chamber with an approximately 70 cm to 6 m diameter, may have an inlet duct with a diameter of approximately 15 cm to 2 m and height of 15 cm to 2 m.
The inlet duct is preferably made of such a material that provides the duct structure with a certain flexibility or elasticity. The duct structure itself may also be flexible.
In accordance with a preferred embodiment of the invention, the inlet duct is formed of two metal cyliners, which are arranged one within the other and which together form a cylindrical double-casing. Between the cylinders is formed an annular slot wherethrough cooling medium is applied. The slot between the cylinders may be either undivided or divided into a plurality of separate sections. The space between the cylinders may, for example, be divided by means of vertical ribs extending from one cylinder to the other, whereby, depending on the quantity of the ribs, two or more separate vertical sections are formed between the cylinders for the cooling medium. Cooling medium may be conducted axially downstream or upstream with respect to the gas flow.
As regards to its structure and material, the inlet duct comprising metal cylinders is elastic. A sudden blow of a hammer on the outer surface of the duct causes a deformation of the duct wall, and the deposits accumualated on the inner surfaces of the duct are disengaged. As it is a cooled duct, the deposits formed on its wall are brittle as such and readily disengageable. Neither do deposits attach to smooth metal surfaces as firmly as to, e.g., refractory-lined surfaces. A stiff, refractory-lined or ceramic duct construction cannot be cleaned with sudden blows of a hammer because the material itself may not be resistant to blows and because a stiff structure does not deform, which would contribute to loosening of the deposit. A blow might also cause the stiff inlet duct to come loose from either end thereof.
An elastic and cooled inlet duct construction may, according to a second embodiment of the invention, be provided by employing a tube which is bended into a spiral or a snail, wherethrough cooling medium is then conducted.
The various layers of the tube bended into a spiral are not fixedly attached to one another, but allow at least some movement of the layers with respect to one another. Removal of the deposits from the inner surface of the inlet duct is effected by, e.g., a blow of a hammer, which is directed to one or more layers of the tube. Consequently, this layer will move with respect to adjacent tube layers, whereby the inner surface of the inlet duct is deformed. As a result of this, the deposits attached to the duct wall come loose. The hammerblow simultaneously causes vibration of the tube, which reflects both ways along the tube in the longitudinal direction. Vibration also loosens the deposits.
Water, steam, air or some other appropriate gas or liquid may be used as a cooling medium in cooled inlet ducts. In that case, also purified and cooled process gas may be used because, in itself, it does not add to the gas load. the most preferable cooling medium is, however, water e.g., because the cooling of the inlet duct may then be in connection with the water/steam circulation of the actual cooling chamber. The cooling medium may be pressurized gas or steam, in which case its heat transfer capacity is better. In that case, the inlet duct is preferably formed of a spirally wound tube, the pressure resistance whereof is higher.
A cooled inlet duct according to the invention has, e.g., the following advantages:
cooling in itself embrittles the deposits accumulating on the duct walls, so they are readily removable by vibration or deformation of the duct; PA1 a metal duct is capable of vibrating and deforming due to a mechanical blow; PA1 an inlet duct of metal is solid and resistant to sudden mechanical force needed for cleaning, and extra particles do not come loose of its walls unlike, for example, of refractory-lined walls; PA1 deposits do not adhere to smooth metal surfaces as easily as to refractory-lined or ceramic surfaces; PA1 a metal duct is light and easy to connect to the cooling chamber and the process itself; PA1 heat may be recovered from a cooled duct.
The present invention is suitable for a great variety of processes. The temperature of the gases issuing from metallurgical processes is normally 700.degree. to 1800.degree. C. before they are conducted to the heat recovery stage, i.e., cooling, where they are normally cooled to a temperature of 350.degree. to 1000.degree. C. The radiation chamber of metallurgical furnaces produces gases of appr. 550.degree. to 1200.degree. C., which are also cooled to appr. 350.degree. to 1000.degree. C. Limestone burning and cement kilns produce gases of appr. 800.degree. to 1000.degree. C., which are cooled to 300.degree. to 500.degree. C. Flue gases from waste incineration furnaces have a relatively low temperature; it may be as low as 300.degree. to 700.degree. C. Still they may contain most different fouling components, which cause trouble until they are cooled to a temperature of appr. 200.degree. to 250.degree. C. Some metallurgical processes also produce gases which have a relatively low temperature but which nevertheless are fouling. Such gases may contain, for example, Pb or Zn compounds melting at a low temperature, and the gases have to be cooled to a relatively low temperature until the formation of deposits is avoided.
The temperature of the inlet duct cooling medium has to be always clearly lower than the eutectic temperature of the molten or vaporizing components contained in the hot gases from the process. This is inevitable for fast cooling of the fouling components which come into contact with the wall surfaces. For example, if water of 20.degree. to 50.degree. C. is used as a cooling medium, the temperature of this water may rise to about 100.degree. C., i.e. without a phase change. The lower the inlet temperature of the cooling medium, the more porous the deposits in the gas duct will be. The temperature of the cooling medium normally rises by about 20.degree.-100.degree. C. in the inlet duct. Often, however, the rise in the temperature is not more than about 20.degree.-30.degree. C. It takes a longer time to cool the deposits in the gas duct by steam, the temperature of which is &gt;200.degree. C. and, consequently, the deposits in the duct become tougher than when using a cooler cooling medium. The gas temperature does not change very much in the inlet duct, usually not more than about 0.5.degree.-25.degree. C.
In the cooling chamber, cooling is effected by a circulating fluidized bed where cold particles are mixed with the gas, thereby lowering the gas temperature immediately below the eutectic temperature of the molten or vaporizing components contained in the gas. Deposits cannot therefore be accumulated on the walls of the cooling chamber.